1. Field of the Invention
This invention relates to a process for controlling the oxygen content of a perovskite oxide, layer perovskite oxide, oxygen-dificient layer perovskite oxide, multilayer perovskite oxide and spinel oxide, thereby providing a superconductive oxide having a critical temperature and/or critical current density, and further to a superconductive device and a process for producing the device.
2. Description of the Related Art
Hitherto, only three kinds of superconductive alloys such as Nb.sub.3 Sn have used for practical applications and their critical temperature exhibiting the superconductivity was at most 23.degree. K. for Nb.sub.3 Ge. Therefore, expensive helium must be used for cooling the superconductive alloys and cooling efficiency is low. For raising up the cooling efficiency, a material exhibiting superconductivity at a higher temperature has been required. Thus, many and intensive studies on various elements, alloys, compounds, ceramics or organic materials have recently been made. Among these materials, it has recently been discovered that perovskite oxides, particularly layer perovskite oxides have a very high critical temperature Tc. For example, the layer perovskide oxide of a La-Ba-Cu-O, La-Sr-Cu-O or Y-Ba-Cu-O type has a Tc of 30.degree. K. or higher, particularly the Y-Ba-Cu-O type oxide has a Tc of 90.degree. K. or higher. Therefore, cheaper liquid hydrogen, liquid neon or liquid nitrogen (one-tenth of the price of liquid helium) can not only be used as a coolant, but also allows their cooling efficiency to be raised up to 20 times that of liquid helium. These superconductive oxides are disclosed in, for example, Physical Review Letters, Vol. 58, No. 4, 1987, pp. 405-407 for the La-Ba-Cu-O type oxides, Physical Review Letters, Vol. 58, No. 4, 1987, pp. 408-410 for the La-Sr-Cu-O type oxides, and NIHON KEIZAI SHIMBUN, Science Column, Mar. 3, 1987 for the Y-Ba-Cu-O type oxides.
These oxides can be prepared from constituents of the oxides by a so-called sintering method. In this method, it is easy to control the proportions of the constituents other than oxygen but hard to control the proportion of oxygen. Particularly, the oxides sintered under the specified conditions of sintering temperature and sintering time alone have a different critical temperature Tc for each of sintered charges. Tc of sintered oxides varies depending upon oxygen partial pressures of sintering atmospheres, even if the oxides are fired at the same temperature for the same period of time. Such great differences in Tc are caused by differences in oxygen concentration between sintered charges. The oxygen concentration of sintered oxides cannot be controlled even by various post-heat treatments. Therefore, prior art has such a problem that high Tc cannot be obtained with good reproducibility. Accordingly, practical and reliable devices utilizing superconductive oxides have not yet been obtained with good reproducibility.
Under the circumstances, some processes for controlling the oxygen content of superconductive oxides have been proposed. Processes for controlling the oxygen content to raise up Tc are described in Z. Phy. B-Condensed Matter 67 (1987), p.507, and Extended Abstracts (The 48th Autumn Meeting, 1987), The Japan Society of Applied Physics, No. 1, p.83. In the former, it is shown that 0.sup.+ of 300 keV was applied at a dose of 5.times.10.sup.16 O.sup.+ /cm.sup.2 to a La-Sr-Cu-O oxides (a film prepared by RF magnetron sputtering) and then this oxide was annealed at 900.degree. C. for 30 minutes, thereby raising up Tc by 4.degree. K. In the latter, it is shown that 0.sup.+ of 0.5 keV was applied at 350.degree. C. and 1.1.times.10.sup.18 ions/cm.sup.2 to a Y-Ba-Cu-O oxide (a film prepared by RF magnetron sputtering), whereby original Tc.sub.offset of 66.degree. K. was raised up to 79.degree. K. On the other hand, processes for controlling the oxygen content to reduce Tc are described in Extended Abstracts (The 48th Autumn Meeting, 1987), The Japan Society of Applied Physics, No. 1, p.84, in which it is shown that H+of 1 keV was applied at 300.degree. K. and 3.5.times.10.sup.17 ions/cm.sup.2 to a Y-Ba-Cu-O oxide (a film prepared by RF magnetron sputtering), thereby conspicuously reducing Tc.sub.offset, which confirms the possibility of forming a weak joint in Josephson elements.
Furthermore, it is also disclosed that a non-superconductive oxide is obtained by applying particle beams or electromagnetic radiations to a superconductive oxide, and a superconductive device in which an applied non-superconductive oxide is formed in a superconductive oxide is also disclosed (see, for example, Proceedings of Symposium S, 1987, Spring Meeting of the MRS, p.8l).
In Proceedings of Symposium S, 1987, Spring Meeting of the MRS, p.8l, lithography and ion-planting were made by applying 0.sup.+ of 500 keV (0.3 to 3 MeV) at 6.times.10.sup.13 ions/cm.sup.2 up to 10.sup.15 /ions/cm.sup.2 to a thin film of YBa.sub.2 Cu.sub.3 O.sub.y prepared by electron beam evaporation, to convert selected areas of the film to a non-superconductive oxide, thus preparing a dc SQUID (Superconducting Quantum Interference Device) which was confirmed to work at 68.degree. K.
In Proc. 18th Int. Conf. on Low Temperature Physics Kyoto, 1987/Japanese Journal of Applied Physics, 26 (1987), Supplement 26-3, fast neutron was applied at 1.3.times.10.sup.18 n/cm.sup.2 (E&gt;0.1 MeV) to La.sub.1.85 Sr.sub.0.15 CuO.sub.4, wehreby Tc was reduced by 3.degree. K. and Jc was raised up to 2 times of 1.2.times.10.sup.4 A/cm.sup.2 in a magnetic field of 4.2K and 2T.
In Physical Review B 36 (1987), pp. 7151-7154, fast neutron was applied up to 8.16.times.10.sup.17 n/cm.sup.2 to single crystals of YBa.sub.2 Cu.sub.3 O.sub.7-.delta., which confirmed that Jc was raised up.